This invention relates to the recovery of oil from limestone, dolomite and other oil-bearing rock composed primarily of carbonates, and is particularly concerned with an extraction process which permits the recovery of oil in substantial quantities.
A large amount of oil exists today in the United States trapped in deposits of limestone and other carbonates located in Southwest Texas. The oil exists in the form of a high viscosity liquid at ambient conditions. As supplies of conventional petroleum are depleted, it will become desirable to recover liquid hydrocarbons from these oil-bearing deposits. It has been suggested that conventional methods of in-situ steam stimulation used in the past with success in recovering oil from tight formations of sand be applied in an attempt to recover heavy oil from limestone deposits. Such methods normally include drilling a series of several boreholes into the formation around a central borehole and introducing high pressure steam into the central borehole. The heat from the steam moves by conduction and convection outward from the central borehole decreasing the viscosity of the trapped oil and forcing it toward the other boreholes from which it is eventually recovered. Attempts to apply such methods to recovering the high viscosity, heavy oils from limestone deposits in Southwest Texas, however, have proven ineffective evidently because the deposits are too shallow to retain high pressure steam which in turn limits the temperature obtainable in the deposit to below that required for good oil production.
In addition to attempting to recover the oil by in-situ steam stimulation, it has been suggested that the oil-bearing limestone be mined and then subjected to pyrolysis in an above-ground retort thereby recovering the oil in a process similar to that used to recover liquid hydrocarbons from oil shale. Such pyrolysis processes normally involve heating the oil-bearing limestone to a temperature between about 700.degree. F. and 900.degree. F. in order to crack and volatilize the oil thereby forcing it out of the interstices of the limestone. Although such a process works effectively, it has some major disadvantages. The primary disadvantages are that the process involves the use of substantial amounts of energy to heat the large volume of limestone rock to high temperatures in order to produce a significant yield of liquids, and that a substantial amount (about 40 wt%) of the oil initially present on the limestone is converted into coke and gas by the high pyrolysis temperatures.